/** * Spins/blocks until node s is matched by a fulfill operation. * * @param s the waiting node * @param timed true if timed wait * @param nanos timeout value * @return matched node, or s if cancelled */ SNode awaitFulfill(SNode s, boolean timed, long nanos) { /* * When a node/thread is about to block, it sets its waiter * field and then rechecks state at least one more time * before actually parking, thus covering race vs * fulfiller noticing that waiter is non-null so should be * woken. * * When invoked by nodes that appear at the point of call * to be at the head of the stack, calls to park are * preceded by spins to avoid blocking when producers and * consumers are arriving very close in time. This can * happen enough to bother only on multiprocessors. * * The order of checks for returning out of main loop * reflects fact that interrupts have precedence over * normal returns, which have precedence over * timeouts. (So, on timeout, one last check for match is * done before giving up.) Except that calls from untimed * SynchronousQueue.{poll/offer} don't check interrupts * and don't wait at all, so are trapped in transfer * method rather than calling awaitFulfill. */ long lastTime = timed ? System.nanoTime() : 0; Thread w = Thread.currentThread(); SNode h = head; int spins = (shouldSpin(s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0); for (; ; ) { if (w.isInterrupted()) s.tryCancel(); SNode m = s.match; if (m != null) return m; if (timed) { long now = System.nanoTime(); nanos -= now - lastTime; lastTime = now; if (nanos <= 0) { s.tryCancel(); continue; } } if (spins > 0) spins = shouldSpin(s) ? (spins - 1) : 0; else if (s.waiter == null) s.waiter = w; // establish waiter so can park next iter else if (!timed) LockSupport.park(this); else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos); } }
/** * Tries to match node s to this node, if so, waking up thread. Fulfillers call tryMatch to * identify their waiters. Waiters block until they have been matched. * * @param s the node to match * @return true if successfully matched to s */ boolean tryMatch(SNode s) { if (match == null && UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) { Thread w = waiter; if (w != null) { // waiters need at most one unpark waiter = null; LockSupport.unpark(w); } return true; } return match == s; }
/** * Spins/blocks until node s is fulfilled. * * @param s the waiting node * @param e the comparison value for checking match * @param timed true if timed wait * @param nanos timeout value * @return matched item, or s if cancelled */ Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) { /* Same idea as TransferStack.awaitFulfill */ long lastTime = timed ? System.nanoTime() : 0; Thread w = Thread.currentThread(); int spins = ((head.next == s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0); for (; ; ) { if (w.isInterrupted()) s.tryCancel(e); Object x = s.item; if (x != e) return x; if (timed) { long now = System.nanoTime(); nanos -= now - lastTime; lastTime = now; if (nanos <= 0) { s.tryCancel(e); continue; } } if (spins > 0) --spins; else if (s.waiter == null) s.waiter = w; else if (!timed) LockSupport.park(this); else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos); } }
/** Puts or takes an item. */ Object transfer(Object e, boolean timed, long nanos) { /* Basic algorithm is to loop trying to take either of * two actions: * * 1. If queue apparently empty or holding same-mode nodes, * try to add node to queue of waiters, wait to be * fulfilled (or cancelled) and return matching item. * * 2. If queue apparently contains waiting items, and this * call is of complementary mode, try to fulfill by CAS'ing * item field of waiting node and dequeuing it, and then * returning matching item. * * In each case, along the way, check for and try to help * advance head and tail on behalf of other stalled/slow * threads. * * The loop starts off with a null check guarding against * seeing uninitialized head or tail values. This never * happens in current SynchronousQueue, but could if * callers held non-volatile/final ref to the * transferer. The check is here anyway because it places * null checks at top of loop, which is usually faster * than having them implicitly interspersed. */ QNode s = null; // constructed/reused as needed boolean isData = (e != null); for (; ; ) { QNode t = tail; QNode h = head; if (t == null || h == null) // saw uninitialized value continue; // spin if (h == t || t.isData == isData) { // empty or same-mode QNode tn = t.next; if (t != tail) // inconsistent read continue; if (tn != null) { // lagging tail advanceTail(t, tn); continue; } if (timed && nanos <= 0) // can't wait return null; if (s == null) s = new QNode(e, isData); if (!t.casNext(null, s)) // failed to link in continue; advanceTail(t, s); // swing tail and wait Object x = awaitFulfill(s, e, timed, nanos); if (x == s) { // wait was cancelled clean(t, s); return null; } if (!s.isOffList()) { // not already unlinked advanceHead(t, s); // unlink if head if (x != null) // and forget fields s.item = s; s.waiter = null; } return (x != null) ? x : e; } else { // complementary-mode QNode m = h.next; // node to fulfill if (t != tail || m == null || h != head) continue; // inconsistent read Object x = m.item; if (isData == (x != null) || // m already fulfilled x == m || // m cancelled !m.casItem(x, e)) { // lost CAS advanceHead(h, m); // dequeue and retry continue; } advanceHead(h, m); // successfully fulfilled LockSupport.unpark(m.waiter); return (x != null) ? x : e; } } }